Thermoformed objects are generally produced from a continuous sheet of thermoformable material that after having been suitably laminated is fed while being still hot between mold and the countermold of a thermoforming machine, and is then thermoformed by bringing the mold and countermold in abutment against each other. One or more thermoformed objects are thus obtained, while the remaining material of the sheet, termed “residual web” in jargon, is removed and suitably recycled.
The thermoformed object or objects thus obtained are extracted from the thermoforming mold by means of a suitable extractor device, or as provided for according to the present invention, they are extracted by the residual web itself, to which they remain anchored at thin cords or “witnesses”; from which they will subsequently be separated in any suitable manner, e.g. via breakage, cutting or the like.
More particularly, according to the most frequently used technology, the male mold and the respective female countermold of a thermoforming press are configured, as is known, so as to perfectly peripherally penetrate each other with sharp edges—likewise a cutting device intended to carry out the cutting or separation of the peripheral edge of each object once thermoformed, and the surrounding residual web (non-thermoformed plastic material sheet), except for some thin, angularly-spaced portions, the “witnesses”. The witnesses must be sufficiently strong to maintain the thermoformed objects anchored to the residual web, but at the same time their size must be as small as possible in order to not leave any visible trace in the finished thermoformed product. In practice, a small enlargement integral with the object always remains at the position of each witness, which is detected by the user of the object at the time of use—both via touch and sight, and constitutes a visible and tactile defect of the thermoformed product, a defect which negatively affects the quality of the finished thermoformed product.
The attachment portions or witnesses are obtained by providing for an incision or recess of predetermined configuration in the sharp edge of the male mold element and/or female countermold at a respective (angular) position along the edge. The incisions are usually executed only on the male mold or on the female countermold. Considering that there are various thermoformable plastic materials used for obtaining thermoformed objects, with very different physical characteristics from each other (tenacity, fragility, ductility, elongation, stretching, etc.), it is necessary to use specific configurations of the incisions for the various materials.
In addition, after a number of cuts or after a fairly long use time interval, the sharp edges at least of the male mold must be restored by means of grinding, since the recess or incisions intended to generate the witnesses are made directly on the cutting or sharp edge of the molds/countermolds, and they undergo a configuration alteration following the grinding of the respective mold. The restoration operation of such configuration after each grinding of the respective mold is rather difficult and quite costly, since it requires the use of special machine tools, which can involve a (press) machine downtime of even several days, with damaging consequences on the productivity of a thermoforming plant. See the patent EP-1 319 493 B1, for example.